Abortion and Breast Cancer: Only fuzzy math can make the ABC link disappear

It looks like "déjà-vu all over again": A supposedly definitive study of immense statistical power, published in a top medical journal, has once again proven the abortion-breast cancer link (ABC link) nonexistent.

This time (March 25 of this year) it was "a collaborative reanalysis of data from 53 epidemiological studies, including 83,000 women with breast cancer from 16 countries".

It was authored by a prestigious group of Oxford researchers, and published in the Lancet, one of the most prominent medical journals in the world. And lead author Valerie Beral wasted no time hyping her group's findings in a frenzy of pre-publication interviews. For example, she told the Associated Press: "The totality of the worldwide epidemiological evidence indicates that pregnancies ended by induced abortion do not have adverse effects on women's subsequent risk of developing breast cancer". "Scientifically, this really is a full analysis of the current data", Beral told the Atlanta Journal-Constitution, suggesting a truly comprehensive review of the data.

To say that the Beral study is seriously flawed and that its conclusions do not stand up to close scrutiny is to understate seriously the magnitude of what is really going on here. For starters, the claim that this is a "full analysis" is flatly false. Let's just do the simple math. We start with 41 studies which showed data on induced abortion and breast cancer, dating as far back as 1957. Then how do we get to 53 sudies? (Actually, the total is 52 studies.) We add 11 studies worth of unpublished data, right? That might be okay, but it wasn't what was done. What Beral et al. actually did was:

Throw out 2 studies for the scientifically appropriate reason that "specific information on whether pregnancies ended as spontaneous or induced abortions had not been recorded systematically for women with breast cancer and a comparison group." Specifically, one such study from Sweden in 1989 used general population statistics for comparison, instead of a control group, and one US study from 1993 ascertained abortions only indirectly, by subtracting the number of children from the number of pregnancies.

Throw out 11 more perfectly good studies for reasons such as: "Principal investigators ... could not be traced" (We can't find Professor Einstein, either. Does that mean we throw out relativity?); "original data could not be retrieved by the principal investigators", "researchers declined to take part in the collaboration", or investigators "judged their own information on induced abortion to be unreliable" (even though it had been published in a prominent medical journal).

Finally, 4 studies' worth of data (one on French women, one on Chinese women, One on Russian women, and one on African-American women) were simply not even mentioned, even though they had been previously published as abstracts or included in other reviews.

That brings the total down from 41 to only 24 studies. Now we add 28 studies worth of unpublished data, and Voilà! We have 52 studies. The fact that the majority of studies have not stood the test of peer review is troubling enough. But a closer look at the excluded studies is even more revealing.

Of the 41 studies which have been previously published, 29 actually show increased risk of breast cancer among women who have chosen abortion. (Epidemiologists call this a "positive association".) 16 of these are statistically significant, which means there is at least a 95% certainty that the results cannot be explained by chance. Getting back to Beral's "full analysis", 10 of the 16 significantly positive studies in the literature were excludied for one of the unscientific reasons cited above. In fact, if we average all of the 15 studies Beral excluded for unscientific reasons, they show an average breast cancer risk increase of 80% among women who had chosen abortion.

So if we just add up all the studies Beral's group decided selectively to include, we get no significant effect of abortion on breast cancer risk. But we haven't even gotten to Beral's main argument yet. She actually divided the included studies into two types; those which used retrospective methods of data collection (i.e., interviews of breast cancer patients v. control subjects), and those which used prospective methods (i.e., medical records taken long before breast cancer diagnosis). The retrospective data-based studies are thought to be less reliable, because, as Beral told the Washington Post, women with breast cancer "are more likely than healthy women to reveal they had an abortion, leading to the conclusion that there are more abortions among this group".

Readers may recognize this "reporting bias" or "response bias" argument, used for over a decade now to dismiss the overwhelming majority of studies (which are retrospective data-based) which reveal an abc link. It is actually a hypothesis worthy of testing. The trouble is, tests for such bias have proven negative over and over and over again in the published literature, in studies as far flung as Japan, the US and Greece. In fact, Beral still reaches back to a 1991 Swedish study, which was the only one ever to claim direct evidence of such "reporting bias". However, that study's conclusion depended upon the assumption--since publicly retracted by the original authors--that breast cancer patients had "overreported" abortions (i.e., reported abortions that had never taken place.)

That brings up another serious flaw in the Beral study, specifically, the exclusion of any published critiques of studies she found acceptable. She included uncritically, for example, data from a 1990 study on Norwegian women which study had found no link. However, in 1998 our own group published a rigorous, mathematical proof that those data were incorrectly compiled, and had actually indicated increased risk among Norwegian women.

Getting back to the reporting bias argument, Beral separately compiled all the studies that used prospective methodology (13 studies) and those that used retrospective methods (39 studies), and found the results to be significantly different. Specifically, the former showed a significant overall 7% decrease in risk with abortion, while the latter showed a significant overall 11% increase in risk.

Beral's conclusion? "We have demonstrated that a certain group of studies (the ones with retrospective data) are unreliable and can't be trusted,", she told the Washington Post." There are only two things wrong with that conclusion.

First, it is completely illogical to leap to the conclusion that, just because there is a difference in the overall results reached by the two types of studies, that the difference is caused by reporting bias. This is especially true in light of the fact that such bias has been repeatedly demonstrated NOT to exist.

Second, at least three of the prospective data-based studies are so seriously flawed themselves as to merit exclusion from the Beral study on the basis of information on abortions having "not been recorded systematically" (see above.) Specifically, these studies included the 1997 Melbye study from Denmark, in which ALL the data on legal abortions before 1973 were missing (only 80,000 abortions on 60,000 women!), A 2001 study in the UK (an Oxford University study, no less), in which over 90% of the abortions in the study population were unrecorded and a 2003 Swedish study, in which data on all abortions after the most recent childbirth were missing. (In Sweden, where abortion is used predominantly to limit family size, that means most of the abortion records for women in the study were missing.) We have published detailed critiques of these studies but, as noted above, these critiques are not cited in Beral's "full analysis".

Another telling aspect of the Beral paper is the graphic depicting the compilation of studies. As noted above, most of the studies which showed significant elevations in risk with induced abortion were inappropriately excluded from the analysis. Then, by combining certain groups of studies and graphing them as "other", it is made to look AS IF NO STUDY EVER FOUND A RELATIVE RISK HIGHER THAN 1.4! In fact, 6 studies (two on Japanese women, two on African-American women, one on Chinese women and one on Australian women) have reported overall relative risks greater than 2.0 (i.e., more than a 100% risk increase with abortion. Finally, I believe an editorial note is in order, because the knee-jerk reaction of so many is to put credence in studies that come from such high places as the Lancet or the New England Journal of Medicine or the National Cancer Institute. As one who has been doing battle on the ABC link in medical and scientific journals and in other public fora for over a decade, nothing has been more obvious to me than the systematic denial of the link from organized science and medicine. In fact, the first study which was specifically designed to "reassure" the public about the safety of abortion vis-à-vis breast cancer was published way back in 1982, and originated from the same cancer research epidemiology unit at Oxford's Radcliffe Infirmary as Beral's "full analysis".

But if the reader would remain skeptical of this writer's observations and conclusions, consider this. It is undisputed--even by Beral herself--that a full-term pregnancy lowers a woman's long term risk of breast cancer, and that this protection is not afforded by a pregnancy that ends in induced abortion. Yet Beral and most of mainstream science and medicine would refuse to say that abortion is therefore a risk factor. In fact, the studious avoidance of characterizing abortion in this way is obvious in the very caption of Beral's summary chart: "Relative risk of breast cancer, comparing the effects of having had a pregnancy that ended as an induced abortion versus effects of never having had that pregnancy." If the same convoluted standard were used in characterizing hormone replacement therapy (HRT) for postmenopausal women, it would also not show up as a risk factor. Specifically, using the same standard would mean comparing postmenopausal women using HRT to premenopausal women of the same age. The conclusion of such a study would be that women using HRT have no greater risk of breast cancer, compared to not having gone into menopause. Instead (and this is no more clearly stated than in Beral's own "Million Woman Study" on HRT and breast cancer, published last year), the study is restricted to postmenopausal women, with those taking HRT thus compared to women who get virtually no estrogen and progesterone at all, from inside or outside. So of course HRT shows up as a risk factor--as well it should.

Everyone knows--including Beral--that a woman who chooses abortion will end up with a higher long term risk of breast cancer than would result from the childbirth choice. Still, unethical and outrageous as it is, it is politically incorrect to inform women seeking abortion of this undeniable truth.